LibJS: Start fleshing out a bytecode for the JavaScript engine :^)

This patch begins the work of implementing JavaScript execution in a
bytecode VM instead of an AST tree-walk interpreter.

It's probably quite naive, but we have to start somewhere.

The basic idea is that you call Bytecode::Generator::generate() on an
AST node and it hands you back a Bytecode::Block filled with
instructions that can then be interpreted by a Bytecode::Interpreter.

This first version only implements two instructions: Load and Add. :^)

Each bytecode block has infinity registers, and the interpreter resizes
its register file to fit the block being executed.

Two new `js` options are added in this patch as well:

`-d` will dump the generated bytecode
`-b` will execute the generated bytecode

Note that unless `-d` and/or `-b` are specified, none of the bytecode
related stuff in LibJS runs at all. This is implemented in parallel
with the existing AST interpreter. :^)

Userland/Libraries/LibJS/Bytecode/Generator.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/OwnPtr.h>
+#include <LibJS/Forward.h>
+
+namespace JS::Bytecode {
+
+class Generator {
+public:
+    static OwnPtr<Block> generate(ASTNode const&);
+
+    Register allocate_register();
+
+    template<typename OpType, typename... Args>
+    void emit(Args&&... args)
+    {
+        auto instruction = make<OpType>(forward<Args>(args)...);
+        append(move(instruction));
+    }
+
+private:
+    Generator();
+    ~Generator();
+
+    void append(NonnullOwnPtr<Instruction>);
+
+    OwnPtr<Block> m_block;
+    u32 m_next_register { 1 };
+};
+
+}